Abstract. In a transient simulation of the last deglaciation with a fully coupled model (TraCE-21000), an overshoot of the AtlanticMeridionalOverturningCirculation (AMOC) is simulated and proposed as a key factor for the onset of the Bølling-Allerød (BA) warming event. There is collaborat- ing evidence for an AMOC overshoot at the BA in vari- ous proxy reconstructions although the mechanism govern- ing its behavior is not well understood. Here, we present two new sensitivity experiments to explicitly illustrate the im- pact of North Atlantic – GIN Sea exchange on the AMOC’s deglacial evolution. Results show that this oceanic exchange dominates the convection restarting in the GIN Sea, the oc- currence of the AMOC overshoot, and the full BA warming.
Abstract. Paleorecords from distant locations on the globe show rapid and large amplitude climate variations during the last glacial period. Here we study the global cli- matic response to different states of the AtlanticMeridionalOverturningCirculation (AMOC) as a potential explana- tion for these climate variations and their possible connec- tions. We analyse three glacial simulations obtained with an atmosphere-ocean coupled general circulation model and characterised by different AMOC strengths (18, 15 and 2 Sv) resulting from successive ∼ 0.1 Sv freshwater perturbations in the North Atlantic. These AMOC states suggest the ex- istence of a freshwater threshold for which the AMOC col- lapses. A weak (18 to 15 Sv) AMOC decrease results in a North Atlantic and European cooling. This cooling is not ho- mogeneous, with even a slight warming over the Norwegian Sea. Convection in this area is active in both experiments, but surprisingly stronger in the 15 Sv simulation, which appears to be related to interactions with the atmospheric circulation and sea-ice cover. Far from the North Atlantic, the climatic response is not significant. The climate differences for an AMOC collapse (15 to 2 Sv) are much larger and of global extent. The timing of the climate response to this AMOC collapse suggests teleconnection mechanisms. Our analy- ses focus on the North Atlantic and surrounding regions, the tropical Atlantic and the Indian monsoon region. The North Atlantic cooling associated with the AMOC collapse induces a cyclonic atmospheric circulation anomaly centred over this
Based on marine proxy data, it has been inferred that the AtlanticMeridionalOverturningCirculation (AMOC) was significantly stronger in the mPWP compared to today (Raymo et al., 1996; Ravelo and Andreasen, 2000; Frank et al., 2002; Frenz et al., 2006; Dowsett et al., 2009; McKay et al., 2012). A stronger AMOC could have contributed to en- hanced northward heat transport, thus explaining the remark- able warming in the North Atlantic at the time (Dowsett et al., 1992, 2009; Lawrence et al., 2010; Naafs et al., 2012). How- ever, the control of the AMOC on transport of heat to high latitudes, and thereby high-latitude ocean surface tempera- ture, is questionable (e.g. Wunsch, 2005). A strong AMOC has also been used to explain a weak Atlanticmeridional δ 13 C gradient during the mid-Pliocene (Raymo et al., 1996; Ravelo and Andreasen, 2000). However, the observed weak Atlantic δ 13 C gradient in the mPWP and its relationship to AMOC strength is unclear (Hodell and Venz-Curtis, 2006).
Abstract. As an extreme scenario of dynamical sea level changes, regional sea surface height (SSH) changes that oc- cur in the North Atlantic due to an abrupt weakening of the Atlanticmeridionaloverturningcirculation (AMOC) are simulated. Two versions of the same ocean-only model are used to study the effect of ocean model resolution on these SSH changes: a high-resolution (HR) strongly eddying ver- sion and a low-resolution (LR) version in which the effect of eddies is parameterised. The weakening of the AMOC is induced in both model versions by applying strong freshwa- ter perturbations around Greenland. A rapid decrease of the AMOC in the HR version induces much shorter return times of several specific regional and coastal extremes in North At- lantic SSH than in the LR version. This effect is caused by a change in main eddy pathways associated with a change in separation latitude of the Gulf Stream.
With the data from the model output a sensitivity analysis is done for the sea surface salinity, mixed layer depth, which is an indicator for the amount of convection, and the Atlanticmeridionaloverturningcirculation. To find out how the sea surface anomaly propagates the correlations of the anomaly of each grid point with the anomaly at Denmark strait is calculated and maps with different time lags are created. Also the spatial changes of the mixed layer depths are plotted, this is done to see whether the convection regions change in location, this can be related to the change in sinking locations. Finally the sinking is calculated for different regions for each model run, this shows where the amount of sinking changes the most and where the decrease of AMOC strength originates from.
Abstract. The Atlanticmeridionaloverturningcirculation (AMOC) has been observed continuously at 26 ◦ N since April 2004. The AMOC and its component parts are mon- itored by combining a transatlantic array of moored in- struments with submarine-cable-based measurements of the Gulf Stream and satellite derived Ekman transport. The time series has recently been extended to October 2012 and the results show a downward trend since 2004. From April 2008 to March 2012, the AMOC was an average of 2.7 Sv (1 Sv = 10 6 m 3 s −1 ) weaker than in the first four years of observation (95 % confidence that the reduction is 0.3 Sv or more). Ekman transport reduced by about 0.2 Sv and the Gulf Stream by 0.5 Sv but most of the change (2.0 Sv) is due to the mid-ocean geostrophic flow. The change of the mid-ocean geostrophic flow represents a strengthening of the southward flow above the thermocline. The increased south- ward flow of warm waters is balanced by a decrease in the southward flow of lower North Atlantic deep water below 3000 m. The transport of lower North Atlantic deep water slowed by 7 % per year (95 % confidence that the rate of slowing is greater than 2.5 % per year).
We analyze the multi-earth system model responses of ocean temperatures and the AtlanticMeridionalOverturningCirculation (AMOC) under an idealized solar radiation management scenario (G1) from the Geoengineering Model Intercomparison Project. All models simulate warming of the northern North Atlantic relative to no geoengineering, despite geoengineering substantially offsetting the increases in mean global ocean temperatures. Increases in the temperature of the North Atlantic Ocean at the surface ( ∼ 0.25 K) and at a depth of 500 m ( ∼ 0.10 K) are mainly due to a 10 Wm 2 reduction of total heat ﬂ ux from ocean to atmosphere. Although the AMOC is slightly reduced under the solar dimming scenario, G1, relative to piControl, it is about 37% stronger than under abrupt4 CO 2 . The reduction of the AMOC under G1 is mainly a response to the heat ﬂ ux change at the northern North Atlantic rather than to changes in the water ﬂ ux and the wind stress. The AMOC transfers heat from tropics to high latitudes, helping to warm the high latitudes, and its strength is maintained under solar dimming rather than weakened by greenhouse gas forcing acting alone. Hence the relative reduction in high latitude ocean temperatures provided by solar radiation geoengineering, would tend to be counteracted by the correspondingly active AMOC circulation which furthermore transports warm surface waters towards the Greenland ice sheet, warming Arctic sea ice and permafrost.
Analyses of meridional transport time series from the Rapid Climate Change–MeridionalOverturningCirculation (RAPID MOC) array at 26 8 N and from Argo float and altimetry data at 41 8 N reveal that, at semiannual and longer time scales, the contribution from the western boundary dominates the variability of the North Atlanticmeridionaloverturningcirculation (MOC), defined as the transport in the upper 1000 m of the ocean. Because the variability of the western boundary contribution is associated with a geostrophic overturning, it is reflected in independent estimates of transports from gradient of ocean bottom pressure (OBP) relative to and below 1000 m on the continental slope of the western boundary at three nominal latitudes (26 8 , 39 8 , and 42.5 8 N). Time series of western meridional transports relative to and below 1000 m derived from the OBP gradient, or equivalently derived from the transport shear profile, exhibit approxi- mately the same phase relationship between 26 8 and 39 8 –42.5 8 N as the western contribution to the geostrophic MOC time series do: the western geostrophic MOC at 41 8 N precedes the MOC at 26 8 N by approximately a quarter of an annual cycle, resulting in a zero correlation at this time scale. This study therefore dem- onstrates how OBP gradients on basin boundaries can be used to monitor the MOC and its meridional coherence.
The analysis shows that much of the variability in the AMOC at 26 ◦ N can be related to the wind strength over the North Atlantic, through mechanisms lagged on differ- ent timescales. At ∼ 1-year lag the January–June difference of mean sea level pressure between high and mid-latitudes in the North Atlantic explains 35–50 % of the interannual AMOC variability (with negative correlation between wind strength and AMOC). At longer lead timescales ∼ 4 years, strong (weak) winds over the northern North Atlantic (specif- ically linked to the NAO index) are followed by higher (lower) AMOC transport, but this mechanism only works in the 1/4 ◦ model. Analysis of the density correlations sug- gests an increase (decrease) in deep water formation in the North Atlantic subpolar gyre to be the cause. Therefore an- other 30 % of the AMOC variability at 26 ◦ N can be related to density changes in the top 1000 m in the Labrador and Irminger seas occurring ∼ 4 years earlier.
work. In contrast, we find that assimilation of actual upper layer hydrographic data, even with the Argo dataset, is not sufficient to constrain the Atlantic AMOC at 26.5 ◦ N. A ma- jor reason for this is because the assimilation fails to con- strain the deep density gradients below 2000 m and those di- rectly along the western boundary. A consequence of this is that circulation changes resulting from assimilation are reflected in gyre transports rather than AMOC transports. To assess the impact of modifying the deeper water masses, we did perform some preliminary experiments assimilating the RAPID array data itself into the model (not shown). However, when the deep RAPID array observations are as- similated using standard Gaussian covariance scales, large changes in dynamic height can develop around the RAPID moorings resulting in a deterioration of the local circulation. A more careful approach may thus be needed, for exam- ple, involving boundary specific covariances associated with lower NADW variability and bias.
Abstract. We compare the variability of the Atlantic merid- ional overturningcirculation (AMOC) as simulated by the coupled climate models of the RAPID project, which cover a wide range of resolution and complexity, and observed by the RAPID/MOCHA array at about 26 ◦ N. We analyse variabil- ity on a range of timescales, from five-daily to interannual. In models of all resolutions there is substantial variability on timescales of a few days; in most AOGCMs the ampli- tude of the variability is of somewhat larger magnitude than that observed by the RAPID array, while the time-mean is within about 10 % of the observational estimate. The am- plitude of the simulated annual cycle is similar to observa- tions, but the shape of the annual cycle shows a spread among the models. A dynamical decomposition shows that in the models, as in observations, the AMOC is predominantly geostrophic (driven by pressure and sea-level gradients), with both geostrophic and Ekman contributions to variability, the latter being exaggerated and the former underrepresented in models. Other ageostrophic terms, neglected in the obser- vational estimate, are small but not negligible. The time- mean of the western boundary current near the latitude of the RAPID/MOCHA array has a much wider model spread than the AMOC does, indicating large differences among mod- els in the simulation of the wind-driven gyre circulation, and its variability is unrealistically small in the models. In many
As previously suggested, melting of floating ice shelves due to subsurface ocean warming could provide an impor- tant feedback to an initial AMOC weakening, accelerating ice sheet mass loss due to their buttressing effect on upstream ice, and adding freshwater that would push the AMOC into a very weak mode (Marshall and Koutnik, 2006; Alvarez-Solas et al., 2010; Marcott et al., 2011). Based on Pa / Th mea- surements at Bermuda Rise, Böhm et al. (2015) and Henry et al. (2016) have both recently argued that AMOC shut- downs occurred exclusively during Heinrich stadials. If so, this would be consistent with AMOC shutdowns having oc- curred only when abundant freshwater was released from the Laurentide ice sheet, amplifying an initial, non-freshwater- forced AMOC weakening. As such, the hosed–unhosed ex- periment may provide a good analogue for Heinrich stadials. Thus, perhaps the question of whether or not a Heinrich event occurred in response to an AMOC interruption had as much to do with the susceptibility of the Laurentide ice sheet to collapse as with the nature of the initial AMOC interruption itself. In turn, the degree to which consequent ice sheet melt- ing altered ocean circulation may have depended on where the freshwater was discharged, including how much of the freshwater was input to the ocean as sediment-laden hyper- pycnal flows (Tarasov and Peltier, 2005; Roche et al., 2007). Although they only occur in our model under an unreal- istic combination of boundary conditions, the spontaneous nature of the unhosed oscillations allows a powerful com- parison to be made with the more typical freshwater-hosed simulations of AMOC weakening. When hosed and un- hosed simulations are compared, the general features of the atmospheric and oceanic responses are remarkably robust. Background climate state introduces as much variability into the the response as the contrast between spontaneous and forced AMOC weakening. These robust features are there- fore likely to reflect consistent dynamical changes related to the AMOC interruption and its coupling with sea ice and atmospheric changes, independent of the ultimate cause of the AMOC interruption. Some aspects of the climate system show greater sensitivity to the magnitude of AMOC weak- ening than others. Of the variables examined here, tropical precipitation showed the strongest sensitivity to an intensifi- cation of AMOC interruption under additional hosing.
In agreement with this speculative statement, the impact of the time-varying forcing on the AMOC is not linear. For the volcanic forcing, we find no significant response of the overturningcirculation at all, in contrast to Stenchikov et al. (2009). A possibility for this difference could be shortcom- ings of the representation of the dynamical response in the atmosphere to volcanic aerosols in the CCSM3 (Stenchikov et al., 2006), which are also found in our simulations (not shown). As for the volcanic forcing, there is no clear re- lationship between the RF variations and the AMOC inten- sity in our simulations. Other studies report a negative im- pact of strong RF changes on the AMOC, both for changes of the solar irradiance (e.g., Goosse and Renssen, 2006) and GHGs (e.g., Bryan et al., 2006). However, this generally ap- plies to RF changes that are stronger than the ones during the last millennium. Extending our transient simulations to the 21st century under the A2 scenario, a decrease of the AMOC strength of 17% is found similar to the reduction reported in Bryan et al. (2006) (not shown). Thus, we conclude that the RF changes in the last millennium are too weak to cause a
For the experiments for Target 2, the warm deep water of the reference state also helped to maintain the strong mix- ing. Without any salinity constraints, however, the surface water became too light (too fresh), because there was less evaporation with the colder SST and freshwater was also ad- vected from the south (Fig. 3). This effect caused the overly weak AMOC in E2-2, E2-5, and E2-7. Although the same mechanism worked to some extent also in E2-1, strong warm anomalies in the deep North Atlantic remained due to the lack of deep-ocean data, which caused continuing strong convection similar to the case of Target 1. By analogy with E1-5 and E1-6, and because of the very small contribution of the deep-ocean data, one might expect that E2-5 would have a similar problem as E2-1, but the SST was adjusted in a much larger area in E2-5 than in E2-1 so that more fresh sur- face water was created. The AMOC was weaker because the excess of freshwater outweighed the effects of low abyssal densities in the absence of deep-ocean data.
creased North Atlantic deep-water formation and increased influence of southern-sourced deep waters in the Atlantic during Heinrich stadials (Elliot et al., 2002; McManus et al., 2004; Skinner et al., 2003; Vidal et al., 1997). More- over, climate models are able to reproduce the bipolar see- saw pattern characterizing millennial-scale glacial variabil- ity through variations of the strength of the Atlantic merid- ional overturningcirculation (AMOC) in response to fresh- water forcings (Ganopolski and Rahmstorf, 2001). However, recent studies show that a shallow circulation cell could have been still active during HSs (Bradtmiller et al., 2014; Gher- ardi et al., 2009; Lynch-Stieglitz et al., 2014; Roche et al., 2014; Wary et al., 2015), suggesting that Greenland temper- ature millennial-scale variability might be related to more complex changes in Atlanticcirculation than simply switch- ing between “on” and “off” circulation modes. A better un- derstanding of the vertical layout and flow rate of the wa- ter masses constituting the AMOC during the last glacial is therefore needed to assess the relationship between AMOC and glacial millennial-scale variability.
The temporal variability and the vertical structure of the transports derived from EB1 and EBH have different char- acteristics. The transports derived from EB1 show much less energy at periods shorter than 50 days, compared to the trans- ports derived from EBH. The leading EOF transport modes show that the vertical shear of the transport arising from EB1 and EBH is especially different in the upper 1000 m. This points to different dynamics governing the density fluctua- tions at EB1 and EBH. Kanzow et al. (2009b) show that the local wind forcing is very different, and much weaker, at EB1 than EBH. Hence, local coastal wind forcing appears to play an important role in setting the variability at EBH. At EB1, the deep-reaching density anomalies may be linked to mesoscale eddies associated with the open ocean circulation. Contrary to the original planning (Marotzke et al., 2002), measurements at EB1 and EBH cannot serve as a backup for each other: densities need to be measured right at the continental slope to compute the eastern boundary density contribution to the AMOC.
The use of temperature and pressure observations to compute dynamic height was originally motivated by Stommel’s (1947) recognition that the increasingly popular use of the (expendable) bathythermogram, (X)BT, provided a potentially valuable resource for geostrophic velocity computations if density could be derived from the directly measured temperature and pressure. He showed that if a water mass’s known T-S correlation were employed, then the errors in dynamic height were small enough to permit meaningful current computations in certain cases. Emery (1975) along with Emery and Wert (1976) developed Stommel’s (1947) idea systematically, culminating in reference T-S curves for 10º by 10º boxes in the Pacific from 20ºS to 40ºN for computation of dynamic height from XBT temperature and pressure records. Siedler and Stramma’s (1983) study extended the method to the Northeast Atlantic, giving consideration to the relative merits of alternative estimators of density from temperature and pressure with final errors in geostrophic velocities as low as 5%. Concurrently, an equivalent study was under way in the western Atlantic at 26.5ºN but using moored temperature sensors rather than XBT casts for the estimation of dynamic height. Again the error arising from the approximation in salinity was smaller than the natural
Abstract. The oceanic response to volcanic eruptions over the last 1000 years is investigated with a focus on the North Atlantic Ocean, using a fully coupled AOGCM forced by a realistic time series of volcanic eruptions, total solar irradi- ance (TSI) and atmospheric greenhouse gases concentration. The model simulates little response to TSI variations but a strong and long-lasting thermal and dynamical oceanic ad- justment to volcanic forcing, which is shown to be a function of the time period of the volcanic eruptions. The thermal re- sponse consists of a fast tropical cooling due to the radiative forcing by the volcanic eruptions, followed by a penetration of this cooling in the subtropical ocean interior one to five years after the eruption, and propagation of the anomalies toward the high latitudes. The oceanic circulation first ad- justs rapidly to low latitude anomalous wind stress induced by the strong cooling. The AtlanticMeridionalOverturningCirculation (AMOC) shows a significant intensification 5 to 10 years after the eruptions of the period post-1400 A.D., in response to anomalous atmospheric momentum forcing, and a slight weakening in the following decade. In response to the stronger eruptions occurring between 1100 and 1300, the AMOC shows no intensification and a stronger reduc- tion after 10 years. This study thus stresses the diversity of AMOC response to volcanic eruptions in climate models and discusses possible explanations.
Abstract. The role of the Tibetan Plateau (TP) in main- taining the large-scale overturningcirculation in the Atlantic and Pacific is investigated using a coupled atmosphere–ocean model. For the present day with a realistic topography, model simulation shows a strong Atlanticmeridionaloverturningcirculation (AMOC) but a near absence of the Pacific merid- ional overturningcirculation (PMOC), which are in good agreement with the present observations. In contrast, the simulation without the TP depicts a collapsed AMOC and a strong PMOC that dominates deep-water formation. The switch in deep-water formation between the two basins re- sults from changes in the large-scale atmospheric circulation and atmosphere–ocean feedback over the Atlantic and Pa- cific. The intensified westerly winds and increased freshwa- ter flux over the North Atlantic cause an initial slowdown of the AMOC, while the weakened East Asian monsoon cir- culation and associated decreased freshwater flux over the North Pacific give rise to the initial intensification of the PMOC. The further decreased heat flux and the associated increase in sea-ice fraction promote the final AMOC collapse over the Atlantic, while the further increased heat flux leads to the final PMOC establishment over the Pacific. Although the simulations were performed in a cold world, it still im- portantly implicates that the uplift of the TP alone could have been a potential driver for the reorganization of PMOC– AMOC between the late Eocene and early Oligocene.
Fresh water hosing experiments, in which the AtlanticMeridionalOverturningCirculation (AMOC) is perturbed by imposing a fresh water flux, usually in the North Atlantic, are useful numerical experiments to understand the mecha- nisms of climate change related to changes in AMOC. There are several motivations to perform such experiments. First, they are a way of characterising the stability of the AMOC, for a given model and a given set of boundary conditions. This can be done in a systematic manner by computing the hysteresis diagram of the AMOC strength reached at equilib- rium for a range of fresh water hosing values (e.g. Rahmstorf, 1995), which can only be achieved with computationally ef- ficient models, i.e., ocean models, as in Rahmstorf (1995), or Earth system models of intermediate complexity (EMICS, cf. Claussen et al., 2002), such as in Ganopolski and Rahm- storf (2001), Weber and Drijfhout (2007) or Lenton et al. (2007). These models show that there are different AMOC equilibrium states as a function of the fresh water flux, and that for specific boundary conditions/fresh water fluxes, there can be several AMOC equilibria. The state reached by the AMOC in such a case depends on the initial condition of the simulation. The fact that the AMOC shows multiple equilib- ria is important because each of these equilibria has a dis- tinct climatic signature, as illustrated by Manabe and Stouf- fer (1988). Their model had two equilibria, one with a “vig- orous” AMOC, the other with no AMOC. The “no AMOC” simulation showed a cooler North Atlantic ocean and cooler Nordic Seas and a stronger northern Hadley cell, with the ITCZ and associated zone of high precipitation shifted south- ward, particularly in the Atlantic. The potential for multiple equilibria of the AMOC associated with the climatic proper- ties of these equilibria led to the hypothesis that the abrupt climatic changes recorded during glacial times could be ex- plained by switches between the different equilibria.